The current invention relates to the use of tetrabenzylthiuram disulfide/urea curative package for curing elastomers in an environmentally safe method of eliminating undesirable nitrosamines. The unique curative package synergistically increases the cure rate of the tetrabenzylthiuram disulfide accelerator while reducing the amount of accelerator needed for vulcanization.
Vulcanization may be defined as a reaction in the presence of heat where a chemical additive reacts with an elastomer to change it from a plastic, tacky solid to a thermoset, fixed solid with improved strength and elasticity, and increased hardness. The vulcanization reaction is one in which the polymeric rubber molecules are cross-linked by the vulcanizing agent to form a network of macromolecules having less mobility and which have the desired physical properties of a usable rubber product. The type of crosslinking (or vulcanizing) agent will vary with the type of rubber used and the properties desired.
The most commonly used vulcanizing agent is sulfur, as it enters into reactions with the majority of the unsaturated rubbers to produce vulcanizates. Sulfur, in the presence of heat, reacts with adjoining olefinic bonds in the polymeric backbone chains or in pendant chains of two elastomeric molecules to form cross-links between the molecular chains.
Vulcanization, as originally known, required long hours and elevated temperatures. Progress was made in speeding the process and improving the properties of the vulcanized product by using accelerators. Reduction in the time required for vulcanization is generally accomplished by changes in the amounts and types of accelerators used.
An accelerator can be defined as a material which, when added to rubber, will materially reduce the time and temperature necessary to effect vulcanization of rubber with sulfur. Organic accelerators are divided into certain chemical groups, and can be classified as either primary or secondary accelerators based on the compound' activity in elastomeric compounds.
Thiazoles and sulfenamides are known to yield a classical curemeter vulcanization curve with adequate scorch time and are known as primary accelerators. Thiurams, dithiocarbamates, aldehyde amines and guanidines are classified as secondary accelerators. It is usual practice for primary and secondary accelerators to be used in combination.
Activators are compounds which render accelerators more potent. Examples of commonly used activators include zinc oxide used with stearic acid and zinc salts of lauric and related fatty acids used with thiazoles, thiurams, and dithiocarbamates. Other examples of activators include the action of guanidines or aldehydeamines with the thiazoles and the action of aromatic amines with xanthogen disulfides.
The compounds which are the subject of this invention include an improvement to the class of accelerators known as thiuram disulfides. Thiuram disulfides constitute a major class of accelerator which are intermediate in activity between the dithiocarbamates and thiazoles. They are less heat sensitive than the former and are readily processable under standard manufacturing conditions.
Thiuram disulfide accelerators are fast, strong, non-discoloring, and give vulcanized stocks having high tensile strengths, low set and high resilience. They function well in the presence of carbon blacks, acidic fillers, or softeners.
Two of the thiuram disulfides in common use are tetramethylthiuram disulfide(TMTDS) and tetraethylthiuram disulfide(TETDS). These accelerators may be used either as primary or secondary accelerators and are especially useful as secondary accelerators with thiazoles as primary accelerators. Recent concerns over certain nitrosamines has created an incentive to minimize their presence in the workplace. Nitrosamines are formed during vulcanization when the amines generated directly or indirectly from these accelerators combine with nitrogen oxides present in the atmosphere. It is postulated, given the basic structure of the thiuram disulfide accelerators, that decomposition and subsequent reaction with nitrogen oxides present in air (NOx) form nitrosamines of the general formula O.dbd.N--NR2, where R is the tetra substituted moiety on the thiuram disulfide.
The challenge is to retain the advantages of the thiuram disulfide accelerators while eliminating the generation of certain undesirable N-nitrosamines. The accelerator of this invention, known as tetrabenzyl thiuram disulfide, meets that challenge only when combined with urea and sulfur in a unique curative package. Toxicologic studies of this experimental accelerator have shown that dibenzyl nitrosamine (DBNA) that may be formed during a vulcanization side reaction as indicated above is non-carcinogenic, is present in very small amounts, and that an increase in its concentration following vulcanization is not observed. This accelerator, tetrabenzylthiuram disulfide (TBTDS) which can be made by, for example, the process given in U.S. Pat. No. 4,459,424, example 37, and by a general process for the preparation of thiuram disulfides contained therein and in U.S. Pat. No. 4,468,526 has been found to have some processing drawbacks and is not a direct replacement for the commonly used tetraalkyl thiuram disulfides described above. When tetramethyl thiuram disulfide is replaced on a part-to-part basis by TBTDS in rubber compounds, some of the physical properties of the cured elastomers are not as satisfactory as those obtained with tetramethyl thiuram disulfide.
An object of this invention is to reduce the time for curing tetrabenzylthiuram disulfide while imparting excellent physical properties to the cured elastomer. A still further object of this invention is to provide a process for curing elastomers which will not generate any potentially dangerous nitrosamines into the workplace.